The present invention relates to a thermal barrier coating applied to the surface of a superalloy article e.g. a gas turbine engine turbine blade, and to a method of applying the thermal barrier coating. The invention particularly relates to ceramic thermal barrier coatings.
The constant demand for increased operating temperature in gas turbine engines was initially met by air cooling of the turbine blades and turbine vanes and the development of superalloys from which to manufacture the turbine blades and turbine vanes, both of which extended their service lives.
Further temperature increases necessitated the development of ceramic coating materials with which to insulate the turbine blades and turbine vanes from the heat contained in the gases discharged from the combustion chambers, again the operating lives of the turbine blades and turbine vanes was extended.
It is known in the prior art to apply these ceramic coating materials by the thermal, or plasma, spray process onto a suitable bond coating, for example a MCrAlY alloy bond coating, which has been applied to the metallic substrate.
It is also known in the prior art to apply these ceramic coating materials by the physical vapour deposition process onto a suitable bond coating which has an alumina interface layer, for example a MCrAlY alloy bond coating or a diffusion aluminide bond coating, which has been applied to the metallic substrate.
It is also known in the prior art to apply these ceramic coating materials by plasma spraying or physical vapour deposition processes onto an oxide layer on the metallic substrate.
The ceramic thermal barrier coatings deposited by the physical vapour deposition process have benefits over the ceramic thermal barrier coating deposited by plasma spraying. The main benefit is improved thermal shock resistance due to the columnar structure of the ceramic thermal barrier coating produced by the physical vapour deposition process.
One problem associated with thermal barrier coatings produced by physical vapour deposition is that the the thermal conductivity is greater than the thermal conductivity of the same thermal barrier coating produced by plasma spraying.
It is known from International patent application WO9318199A to produce a columnar ceramic thermal barrier coating which comprises a plurality of layers with interfaces between the adjacent layers. Each columnar grain extends perpendicularly to the surface of the metallic article. Each columnar grain has a plurality of layers and adjacent layers have different structures. The interfaces between adjacent layers decreases the thermal conductivity of the ceramic thermal barrier coating. These layers are produced by alternate physical vapour deposition and plasma assisted physical vapour deposition.